The proposed project involves an electrophysiological investigation of the ballistic startle response in the pteropod mollusk Clione limacine. In the long term, we aim to describe both dramatic and subtle locomotory speed changes from the behavioral level to the level of membrane currents of component neurons and muscle cells. The Clione preparation is an excellent model system for examination of locomotory speed changes since a great deal is known about the swim circuitry and swim musculature, and since swimming shows both dramatic and subtle speed changes, in addition to ballistic escape swimming. The startle response/escape swimming is of interest for several reasons. The brief, ballistic startle response utilizes the same musculature as in normal swimming, however, the normal swim pattern generator, along with swim motoneurons are totally inhibited during the startle. In this project, we will examine the sensory activation of the startle, focusing on three newly identified tail mechanosensory neurons. In addition, the nature of the temporary inhibition between startle and swim systems will be described. Finally, we will examine mechanisms responsible for the immediate return to fast swimming at termination of the startle. The superimposition of startle responses on ongoing swimming activity is a problem faced by higher animals, including humans. The unique nature of the Clione startle system should provide insights into how two competing motor systems interact to produce appropriate hierarchical interactions that include temporary inhibition of locomotion by a startle reflex, and the immediate and smooth resumption of locomotion following the startle.